Photonic input/output coupler alignment

ABSTRACT

Optical alignment of an optical connector to input/output couplers of a photonic integrated circuit can be achieved by first actively aligning the optical connector successively to two loopback alignment features formed in the photonic chip of the PIC, optically unconnected to the PIC, and then moving the optical connector, based on precise knowledge of the positions of the loopback alignment features relative to the input/output couplers of the PIC, to a position aligned with the input/output couplers of the PIC and locking it in place.

PRIORITY

This application is a continuation of U.S. patent application Ser. No.15/967,313, filed Apr. 30, 2018, which has been incorporated byreference herein in its entirety.

TECHNICAL FIELD

This disclosure relates to aligning optical connectors with input/outputcouplers of a photonic integrated circuit.

BACKGROUND

Photonic integrated circuits (PICs), as are commonly used, for example,in optical routers and switches, generally include input/output couplersfor optically connecting the PICs to optical fibers or other off-chipoptical connectors. For example, planar waveguides in a PIC may end ingrating couplers that can surface-couple light (via the top surface ofthe photonic chip) into out-of-plane optical fibers. Alternatively, thewaveguides may be edge-coupled to fibers at side faces of the photonicchip. To ensure efficient coupling of light between the opticalconnector (such as an optical fiber or fiber ribbon) and the PIC, thecommunication channel(s) of the optical connector (e.g., the individualfibers of a fiber ribbon) need to be precisely aligned with theinput/output coupler(s) of the PIC. For multi-mode optical signals,where alignment accuracies within 2 μm are sufficient, alignment can beachieved visually based, for instance, on fiducial markers placed on thephotonic chip at accurately known positions relative to the input/outputcouplers. Efficient single-mode coupling, however, relies on accuracieswithin 1 μm or less, which exceeds the performance of visual alignment.Therefore, single-mode fibers or other optical connectors are usuallyaligned to the PIC actively.

During active alignment, light may be coupled from the optical connectorinto an input coupler of the PIC and measured by a detector of the PIC,or, alternatively, light generated by an on-chip light source may becoupled from an output coupler of the PIC into the optical connector andmeasured by an off-chip detector. Either way, by maximizing theintensity of the detected signal as the optical connector is wiggledabout the approximate location of the input/output coupler of the PIC,the alignment can be optimized. Active alignment is, however,time-consuming because it involves powering up the PIC (to enable use ofthe on-chip light source or detector), which entails time to establishthe requisite electrical connections and time waiting for the PIC toreach thermal and optical stability. Accordingly, an alternative methodfor single-mode alignment is desirable.

BRIEF DESCRIPTION OF THE DRAWINGS

Various example embodiments are herein described in conjunction with theaccompanying drawings.

FIG. 1A is a schematic top view of a pair of grating couplers of a PIC,in accordance with various embodiments.

FIG. 1B is a schematic bottom view of an optical connector with twocommunication channels matching the pair of grating couplers in width,in accordance with various embodiments.

FIG. 1C is a schematic side view of the optical connector of FIG. 1B inposition for optical coupling to the pair of grating couplers of FIG.1A, in accordance with various embodiments.

FIG. 2 is a schematic top view of a photonic chip including a pair ofgrating couplers of a PIC and an associated loopback alignment feature,in accordance with various embodiments.

FIG. 3 is a conceptual diagram of an example alignment system using aloopback alignment feature in accordance with various embodiments.

FIG. 4 is a schematic top view of a photonic chip including a pair ofgrating couplers of a PIC and two associated loopback alignmentfeatures, in accordance with various embodiments.

FIGS. 5A-5C are schematic top views of the photonic chip of FIG. 4overlaid with an optical connector in various positions, illustratingthe steps of aligning the optical connector with the pair of gratingcouplers in accordance with various embodiments.

FIG. 6 is a schematic top view of a photonic chip including a pair ofgrating couplers of a PIC and an associated loopback structure with twoloopback alignment features, in accordance with various embodiments.

FIGS. 7A-7C are schematic top views of the photonic chip of FIG. 6overlaid with an optical connector in various positions, illustratingthe steps of aligning the optical connector with the pair of gratingcouplers in accordance with various embodiments.

FIG. 8 is a schematic top view of a photonic chip including an array offour grating couplers of a PIC and two associated loopback alignmentfeatures, in accordance with various embodiments.

FIGS. 9A-9C are schematic top views of the photonic chip of FIG. 8overlaid with an optical connector in various positions, illustratingthe steps of aligning the optical connector with the array of gratingcouplers in accordance with various embodiments.

FIG. 10 is a perspective view of an optical connector in position foroptical coupling to a pair of waveguide edge couplers, in accordancewith various embodiments.

FIG. 11 is a perspective view of an optical connector in position foroptical coupling to an array of six waveguide edge couplers, inaccordance with various embodiments.

FIG. 12 is a schematic top view of a photonic chip including a pair ofwaveguide edge couplers of a PIC and two associated loopback alignmentfeatures, in accordance with various embodiments.

FIGS. 13A-13C are schematic top views of the photonic chip of FIG. 12and an optical connector in various positions, illustrating the steps ofaligning the optical connector the waveguide edge couplers in accordancewith various embodiments.

FIG. 14 is a flow chart illustrating methods of aligning an opticalconnector to input/output couplers of a PIC using loopback alignmentfeatures, in accordance with various embodiments.

FIG. 15 is a flow chart illustrating an example method of manufacturinga photonic chip with input/output couplers of a PIC and associatedloopback alignment features in accordance with various embodiments.

DETAILED DESCRIPTION

Disclosed herein is an approach for aligning optical connectors toinput/output couplers of a PIC formed in a photonic chip that usesloopback alignment features formed in the photonic chip opticallyunconnected to the PIC. A “PIC” is herein understood as a set of optical(and/or electro-optic) components integrated in the photonic chip thatare all optically coupled to each other so as to form a single opticalcircuit. By contrast, the term “photonic chip” is herein used inreference to the physical unit formed by the photonic-chip substrate(e.g., a die cut from a silicon, silicon-on-insulator (SOI), III-V orII-IV or compound, or other wafer, optionally with additional materiallayers deposited thereon) and any components formed in or on thesubstrate, including, but not limited to, the components of the PIC. Aloopback alignment feature as described herein is formed in the samephotonic chip as the PIC whose alignment it serves, but is not itselfpart of that PIC; rather, the loopback alignment feature forms aseparate component that is not optically connected to the PIC.

An optical connector as used herein generally includes at least twooptical communication channels for coupling to at least two respectiveinput/output couplers of the PIC. The end points of the individualcommunication channels of the optical connector generally have fixedrelative distances and positions, matching the fixed relative distancesand positions of the coupling points on the input/output couplers of thePIC to which the optical connector is designed to couple. In variousembodiments, the end points of the communication channels of the opticalconnector and the coupling points of the input/output couplers of thePIC are arranged in arrays. The optical connector may take the form oftwo or more optical fibers, which may be packaged, for example, in afiber ribbon. Other possible embodiments of an optical connector includea fiber-coupling lens array, a lens matrix, an array of siliconwaveguides, another PIC, etc. The input/output couplers of the PIC maybe implemented, for instance, as grating couplers, turning mirrors, orother surface couplers that re-direct light from a waveguide in theplane of the PIC through the top surface of the photonic chip into anout-of-plane fiber or other communication channel of the opticalconnector, or as waveguide edge couplers that couple light through aside face of the photonic chip into an optical communication channel ofthe optical connector in-plane with the PIC.

In general, a loopback alignment feature in accordance herewith isformed by a pair of input/output couplers that are optically connectedin the chip, e.g., via a waveguide. Such a loopback alignment featurecan be actively aligned with an optical connector having at least twocommunication channels (e.g., two optical fibers): light from a lightsource external to the photonic chip can be coupled, via one of thechannels, into one of the couplers (acting as input coupler) of theloopback alignment feature, and after the light has traversed the “loopback” to the other coupler (acting as output coupler) of the loopbackalignment feature, the light can be coupled from the output coupler intoanother one of the optical-connector channels to be measured by adetector external to the photonic chip. Once the optical connector hasbeen actively aligned to the loopback alignment feature, it can besimply moved into alignment with the input/output couplers of the PICbased on knowledge of the position of the loopback alignment featurerelative to the input/output couplers of the PIC. Precise knowledge ofthat relative position may be available, for instance, as a result ofthe photolithographic definition of the loopback alignment feature onthe photonic chip simultaneously with the definition of the input/outputcouplers of the PIC, e.g., using a single photomask. The opticalconnector, once aligned with the input/output couplers, can be locked inplace such that its communication channels are securely coupled to theinput/output couplers of the PIC.

In various embodiments, the alignment process utilizes two loopbackalignment features to facilitate calibrating the coordinate system inwhich the PIC and loopback alignment features are defined relative tothe movement of the optical connector. The optical connector issequentially aligned to first one and then the other loopback alignmentfeature, and the direction in which the optical connector is moved fromone to the other loopback alignment feature is used to determine theorientation of the coordinate system. The two loopback alignmentfeatures may form separate structures in the PIC, or may, alternatively,jointly form a single structure.

Beneficially, by using loopback alignment features as described hereinto align an optical connector with, ultimately, the input/outputcouplers of a PIC, alignment accuracies suitable for single-modecoupling can be achieved without the need to power up the PIC. As aresult, the throughput of photonic chips at the alignment stage of amanufacturing line can be increased substantially, for example, fourfoldin some embodiments. Further reducing cost, the equipment used for(usually automatic) alignment can be greatly simplified, as electronicsfor powering up the device and tooling to establish electricalconnections are no longer needed. In addition, the alignment processdescribed herein can be performed across a range of temperatures,allowing to compensate for thermal expansion, and variations in theprocess resulting from optical device variations between channels can beeliminated. For example, if the PIC is intended to operate between 0 and100° C., the alignment may be done at 50° C. in order to minimize theerror from thermal expansion/contraction mismatch between the PIC andthe connector across the range of 0 to 100° C.

Various example embodiments will now be described with reference to theaccompanying drawings.

FIGS. 1A-1C illustrated the coupling of an example optical connectorwith two communication channels to two grating couplers of a PIC in anexample photonic chip. The two grating couplers may be both inputcouplers or both output couplers, or include both an input coupler andan output coupler. FIG. 1A provides a schematic top view of the examplephotonic chip 100, showing the two grating couplers 102, 104 connectedto associated waveguides 106 of the PIC. For simplicity and clarity, noother components of the PIC are shown. Note, however, that, although notshown, the grating couplers 102, 104 are generally optically connectedvia a path through the PIC. (It is noted that the photonic chip 100 may,in some embodiments, include multiple PICs with multiple respective setsof grating couplers; in this case, there need not be optical connectionsbetween grating couplers of different ones of the PICS.) The distancebetween the two grating couplers—or, more precisely, the distancebetween their respective coupling points, herein taken to be thegeometric centers of the grating couplers 102, 104—has a value specifiedby design (e.g., in the photomask used to create the grating couplers102, 104 on the photonic chip), indicated as distance x in FIG. 1A. Inthe depicted example, each of the grating couplers 102, 104 is apolarization splitting coupler connected to a pair of waveguides 106.The waveguides 106 of each grating coupler 102 or 104 guide light of tworespective orthogonal polarizations into which the grating coupler 102or 104 separates light received from an optical connector when operatingas an input coupler, or which it combines when operating as an outputcoupler. In alternative embodiments, single-polarization gratingcouplers each connected to only one waveguide of the PIC may be used.The grating couplers 102, 104 may be formed by gratings including aplurality of elongate scattering elements. The grating couplers 102, 104may be flared, optionally with hyperbolically shaped sidewalls. Theelongated scattering elements may have curved (e.g., in someembodiments, elliptical) shapes, and the grating widths may be selectedto accommodate a desired optical intensity distribution.

FIG. 1B is a schematic bottom view of the example optical connector 110with two communication channels 112, 114, which may be, for instances,optical fibers (as shown in FIG. 1C). The distance between the endpoints of the communication channels 112, 114 is x, matching thedistance between the pair of grating couplers 102, 104 of the PIC. FIG.1C illustrates, in a schematic side view of the optical connector 110and the photonic chip 100, the lateral alignment of the communicationchannels 112, 114 of the optical connector 110 with the input/outputcouplers 102, 104 on the photonic chip 100 just prior to coupling.

Turning now to FIG. 2, the layout of an example photonic chip 200including a pair of grating couplers 102, 104 of a PIC and an associatedloopback alignment feature 206, as used, in accordance with variousembodiments, to facilitate the alignment illustrated in FIG. 1C, isshown in a schematic top view. The loopback alignment feature 206includes two (polarization-splitting) grating couplers 208, 210 that areoptically connected, via two waveguides 212, 214, into a closed loop.The distance between the coupling points of the grating couplers 208,210 of the loopback alignment feature 206 is equal to the distance xbetween the coupling points of the input/output grating couplers 102,104 of the PIC, and, accordingly, also equal to the distance between thetwo communication channels of an optical connector 110 designed to matewith the input/output grating couplers 102, 104 of the PIC. Further, inaccordance herewith, the position of the loopback alignment feature 206relative to the grating couplers 102, 104 of the PIC, as reflected inthe vector {right arrow over (r)} from the midpoint 216 between thegrating couplers 208, 210 of the loopback alignment feature 206 to themidpoint 218 between the grating couplers 102, 104 of the PIC, is knownaccurately and precisely. During the alignment process, the opticalconnector 110 is first actively aligned to the loopback alignmentfeature 206 by optimizing the optical coupling of the communicationchannels 112, 114 of the optical connector 110 to the grating couplers208, 210 of the loopback alignment feature 206. Following its alignmentto the loopback alignment feature 206, the optical connector 110 ismoved by the vector {right arrow over (r)} into alignment with thegrating couplers 102, 104 of the PIC.

FIG. 3 conceptually illustrates an example alignment system 300 thatfacilitates optical-connector alignment using a loopback alignmentfeature 206. The system 300 includes a mount 302 for securely retainingthe photonic chip 200 and a robot 304 with high-accuracy stages (e.g., athree-axes or six-axes robot) for precisely translating and/or rotatingthe optical connector 110 relative to the photonic chip 200. Further,the system 300 includes a light source 306 and detector 308.Beneficially, the system 300 need not include electronics and tooling toestablish electrical connections and supply electrical power to thedevices of the PIC 305 on the photonic chip 200.

To actively align the optical connector 110 to the loopback alignmentfeature 206, one of the communication channels of the optical connector,say, channel 112, is used to provide an optical signal from the lightsource 306 as input to one of the grating couplers of the loopbackalignment feature 206, say, coupler 208. From (input) coupler 208, theoptical signal travels via the loopback waveguides 212, 214 (not shownin FIG. 3) to the other grating coupler 210, which serves as outputcoupler. Via the second communication channel 114 of the opticalconnector 110, the optical signal output at grating coupler 210 isdelivered to the detector 308 for measurement. The measured signal canbe provided as feedback to the robot 304 (e.g., via a controllercontrolling operation of the robot 304) to cause adjustments of thepositioning of the optical connector 110 that optimize coupling betweenthe communication channels 112, 114 and the grating couplers 208, 210 ofthe loopback alignment feature 206, thereby maximizing the signal.While, for purposes of this alignment process, one of the two connectingwaveguides 212, 214 may suffice, establishing two optical connections asdepicted in FIG. 2 is beneficial in that it enables retaining twopolarizations of the light during the active alignment of an opticalconnector to the loopback alignment feature 206. With an unpolarizedlight source, half of the light would be lost with only one connectionbetween the two grating couplers 208, 210, and with a polarized lightsource, there is a risk that all of the light may be lost (depending onthe polarization state).

When the alignment to the loopback alignment feature 206 is complete,the robot 304 is then operated to move the optical connector 110 into aposition aligned with the grating couplers 102, 104 of the PIC 305 onthe photonic chip 200. Additional apparatus (not shown in FIG. 3) mayaid in locking the optical connector 110 in place with epoxy, solder,laser weld, or by other means known to those of ordinary skill in theart.

FIG. 4 shows the layout of an example photonic chip 400 including a pairof grating couplers 102, 104 of a PIC and two associated loopbackalignment features 406, 408, in accordance with various embodiments. Thetwo loopback alignment features 406, 408 may (but need not) bestructurally similar or identical to each other (and to the loopbackalignment feature 206 of the photonic chip 200 of FIG. 2). Inparticular, each of the loopback alignment features 406, 408 may have apair of grating couplers (410, 412, and 414, 416) whose mutual distanceis equal to the mutual distance x between the grating couplers 102, 104of the PIC. The positions of the loopback alignment features 406, 408are accurately known relative to each other and to the grating couplers102, 104 of the PIC, and may be specified, for instance, by any twovectors among a vector {right arrow over (r)}₁ between the (midpointsbetween grating couplers 410, 412 and 414, 416 of the) loopbackalignment features 406, 408, a vector {right arrow over (r)}₂ betweenthe (midpoints between grating couplers 414, 416 of the) loopbackalignment feature 408 and the pair of grating couplers 102, 104 of thePIC, and the vector {right arrow over (r)} between the (midpointsbetween grating couplers 410, 412 of the) loopback alignment feature 406and the pair of grating couplers 102, 104 of the PIC. The use of asecond feedback alignment feature 408 may serve to calibrate thecoordinate system in which the PIC and loopback alignment features 406,408 are defined, in particular, to determine the orientation of thatcoordinate system relative to the coordinate system in which themovement of the optical connector 110 (by, e.g., the robot 304) isspecified.

FIGS. 5A-5C are schematic top views of the photonic chip 400 of FIG. 4overlaid with an optical connector 110 in various positions,illustrating the steps of aligning the optical connector 110 with thepair of grating couplers 102, 104 of a PIC in accordance with variousembodiments. FIG. 5A depicts the two communication channels 112, 114 ofthe optical connector 110 aligned with the grating couplers 410, 412 ofthe first loopback alignment feature 406. FIG. 5B depicts the subsequentalignment of the two communication channels 112, 114 with the gratingcouplers 414, 416 of the second loopback alignment feature 408. Thetranslation of the optical connector 110, by vector {right arrow over(r)}₁, from the first aligned state, shown in FIG. 5A, to the secondaligned state, shown in FIG. 5B, determines the orientation of thephotonic chip layout (and its associated coordinate system) within thecoordinate system of the apparatus (e.g., robot 304) that moves theoptical connector. Once the coordinate system associated with the chiplayout has been thus calibrated, the optical connector can simply bemoved by the known vector {right arrow over (r)}₂ from the secondloopback alignment feature 408 to the grating couplers 102, 104 of thePIC, resulting in the final position of the optical connector 110 thatis shown in FIG. 5C. In this final position, the optical connector 110is secured in place.

In the absence of a second loopback alignment feature, the relativeorientation between the coordinate system associated with the chiplayout and the coordinate system associated with the movement of therobot 304 and optical connector 110, respectively, may, at least inprinciple, be inferable from the orientation of the optical connector110 relative to the coordinate system in which it moves, e.g., asspecified with a vector between the end points of the communicationchannels 112, 114 of the optical connector 110, When the opticalconnector 110 is aligned with the loopback alignment feature 206, thevector between these end points of the optical connector 110 coincideswith the vector between the grating couplers of the loopback alignmentfeature 206, which, in turn, has a known direction and length within thechip layout. Thus, alignment of the optical connector 110 with theloopback alignment feature 206 may serve to calibrate the coordinatesystem in which the PIC and loopback alignment feature are defined.However, in practice, the orientation of the optical connector within,for instance, a grabber of the robot 304, is usually not be known withsufficient precision. This orientation can be approximated using avision system to recognize alignment features on the PIC and theconnector, but this is not necessarily precise. The use of two loopbackalignment features is, in this case, beneficial to resolve theorientation uncertainty and avoid alignment errors. This is particularlyimportant in cases where there are no repeatable, accurate features onthe PIC's visible surface that the vision system can recognize, or if anaccurate vision system is not available. For example, by using the twoloopback alignment features for calibration of the PIC coordinate systemwith the robot's coordinate system, the accuracy of the vision systemand the accuracy of the initial position and orientation of the robotrelative to the PIC can be relaxed to tens of microns instead of needingto be smaller than about five microns.

As shown in FIG. 4, the two loopback alignment features may beimplemented as separate, optically unconnected structures. As analternative embodiment, FIG. 6 illustrates the layout of an examplephotonic chip 600 in which the two loopback alignment features areinstead provided by a single loopback structure 602. This structure 602includes, in the depicted example, three grating couplers 604, 606, 608uniformly spaced in a linear arrangement, with waveguides 610, 612connecting neighboring pairs of waveguides, and a third waveguide 614(which may be omitted in some embodiments) connecting the outer gratingcouplers 604, 608 of the linear arrangement directly to each other toform a closed loop. Light input to any one of the grating couplers willpropagate to the other two grating couplers. The pair of adjacentgrating couplers 604, 606, along with the waveguide 610 connecting them,forms one loopback alignment feature 616, and the pair of adjacentgrating couplers 606, 608, along with the waveguide 612 connecting them,forms another loopback alignment feature 618. The two loopback alignmentfeatures 616, 618 share a common width (that is, distance between therespective pair of grating couplers), which may, by design, be equal tothe distance x between the input/output couplers 102, 104 of the PIC onthe photonic chip 600.

FIGS. 7A-7C are schematic top views of the photonic chip 600 of FIG. 6overlaid with an optical connector 110 in various positions,illustrating the steps of aligning the optical connector 110 using theloopback structure 602. As can be seen, alignment based on two loopbackalignment features of a single loopback structure is achieved in thesame general manner as alignment based on two separate alignmentfeatures (as shown in FIGS. 5A-5C). In FIG. 7A, the optical connector110 is shown in alignment with the first loopback alignment feature 618formed by the middle grating coupler 606 and its neighbor to the right,grating coupler 608. To move the optical connector 110 into alignmentwith the second loopback alignment feature 616 as shown in FIG. 7B, theoptical connector 110 is simply shifted to the left (along the lineconnecting the three grating couplers 604, 606, 608) by the width x of apair of adjacent couplers, corresponding to a vector {right arrow over(r)}₁. These two alignment steps achieve calibrating the coordinatesystem associated with the photonic-chip layout and locating the opticalconnector 110 within that coordinate system. The optical connector 110can then be brought into alignment with the input/output couplers 102,104 of the PIC by moving it by the vector {right arrow over (r)}₂ fromthe second loopback alignment feature 616 to the pair of input/outputcouplers 102, 104.

The above-described examples all illustrate the alignment of twocommunication channels 112, 114 of an optical connector 110 to a singlepair of input/output couplers of a PIC. The process can bestraightforwardly generalized, however, to the alignment of an opticalconnector with two or more communication channels to two or morerespective input/output couplers, which may be arranged in a one- ortwo-dimensional array. The input/output couplers of the PIC may includeany combination of one or more input couplers operating to receiveoptical signals and/or one or more output couplers operating to transmitoptical signals, that is, they may be all input couplers, all outputcouplers, or a combination of both.

FIG. 8 is a schematic top view of an example photonic chip 800 includingan array 802 of eight grating couplers of a PIC and two associatedloopback alignment features 804, 806, in accordance with variousembodiments. The array 802 of grating couplers includes two parallel,horizontally aligned rows of four grating couplers each. Within eachrow, the grating couplers are uniformly spaced, and the spacing is thesame for both rows. In other words, the gratings are arranged in thearray with a constant pitch. Denoting the pitch, that is, the distancebetween any two adjacent grating couplers within a row, by x, the outergrating couplers 808, 810 and 812, 814 in the two rows have a distanceof 3x. The loopback alignment features 804, 806 each include two gratingcouplers (820, 822 and 824, 826) connected via two waveguides into aclosed loop. The distance between the two grating couplers within eachof the alignment features 804, 806 is, in the depicted example, equal to3x, such that, for the active alignment of an optical connector to theloopback alignment feature 804, 806, a pair of communication channelsconfigured to mate with the outer grating couplers 808, 810 or 812, 814of the PIC is used.

FIGS. 9A-9C illustrate the alignment process for an array 802 of gratingcouplers in schematic top views of the photonic chip 800 of FIG. 8overlaid with an optical connector 900 in various positions. The opticalconnector 900 includes two rows of four communication channels each,spaced and arranged in the same manner as the input/output gratingcouplers of the array 802 on the photonic chip 800. Only a pair of outercommunication channels 902, 904 of one of the rows is used forsequential alignment with the loopback alignment features 804, 806, asillustrated in FIGS. 9A and 9B, respectively. Based on the knownrelative positions between the two loopback alignment features 806, 804(e.g., as reflected in a vector {right arrow over (r)}₁ connecting themidpoints between their respective pairs of gratings 824, 826 and 820,822) and between at least one of the loopback alignment features 804,806 and the array 800 of PIC grating couplers (e.g., as reflected in avector {right arrow over (r)}₂ from the midpoint between gratings 820,822 of loopback alignment feature 804 to the midpoint between the outergrating couplers 808, 810 of the array 800), the optical connector 900can be aligned with the array 802 of grating couplers of the PIC. If theeight communication channels of the optical connector 900 are arrangedprecisely like the input/output grating couplers of the array 802 of thePIC, accurate alignment of the pair of outer communication channels 902,904 with the pair of outer grating couplers 808, 810 inherently alsoensures the proper alignment of all other communication channels to therespective grating couplers.

As will be readily appreciated by those of ordinary skill in the artgiven the benefit of the present disclosure, the distance between thetwo grating couplers within a loopback alignment feature need notnecessarily match the width of a row within the array of input/outputcouplers of the PIC (which is, in the example of FIGS. 8-9C, 3x). Forexample, the loopback alignment feature may have grating couplers spacedat a distance x corresponding to the distance between two adjacentinput/output couplers of the PIC, that is, to the pitch of the array ofinput/output couplers. In general, for any array of input/outputcouplers of a given pitch, the distance between the two grating couplersof the loopback alignment feature may be any integer multiple of thatconstant pitch, up to a maximum value corresponding to the width of thearray. Even more generally, the positions of the couplers of theloopback alignment feature relative to each other can match the relativemutual positions of any two input/output couplers of the PIC (even twocouplers of different rows). Further, as with the alignment of anoptical connector with two communication channels to a single pair ofinput/output couplers of the PIC, the alignment of larger arrays ofcommunication channels to corresponding arrays of input/output couplersmay also utilize loopback alignment features provided in a singlestructure rather than by two separate structures.

Furthermore, while FIGS. 1A-9C all illustrate the alignment to gratingcouplers, the described alignment processes, structures, andconfigurations can readily be applied to other types of couplers, suchas, for example, turning mirrors. In practice, the same type of couplerwill usually be employed for all input/output couplers of the PIC aswell as the couplers of the loopback alignment feature(s). In principle,however, it is also possible to use different types of couplers for theloopback alignment feature(s) than are used in the PIC, and/or even tomix coupler types within the PIC or within a loopback alignment feature.

The principles discussed above are, moreover, not limited to surfacecoupling, but can be applied to edge coupling as well. To illustrate,FIG. 10 shows, in a perspective view, a photonic chip 1000 including, inthe plane of the top surface 1002 of the photonic chip 1000, twowaveguides 1004, 1006 terminating at a side face 1008 of the chip 1000.These waveguides 1004, 1006 serve as waveguide edge couplers to anoptical connector 110 oriented with its communication channels 112, 114in the plane of the PIC. FIG. 10 shows the optical connector 110 inalignment (albeit not yet coupled) with the coupling points 1016, 1018provided by the waveguide edge couplers 1004, 1006 in the side face1008.

FIG. 11 illustrates a variant of such edge-coupling structures, showinga photonic chip 1100 that includes six waveguides 1102 embedded in twolayers of the photonic chip 1100 parallel to the top surface 1104 (withthree waveguides 1102 in each of the two layers). At the side face 1106,the six waveguides 1102 form a two-by-three array of coupling points1108. An optical connector 1110 with a corresponding array of sixcommunication channels 1112 is shown in alignment with the couplingpoints 1108.

The use of loopback alignment features for the alignment of an opticalconnector to waveguide edge couplers is illustrated in FIGS. 12-13C forthe example of a single pair of waveguide edge couplers. FIG. 12provides a schematic top view of a photonic chip 1200 including a pairof waveguide edge couplers 1202, 1204 of a PIC and two associatedloopback alignment features 1206, 1208, in accordance with variousembodiments. Each of the loopback alignment features 1206, 1208 forms aU-turn waveguide terminating, at both ends, at the same side face 1210of the photonic chip 1200 as the waveguide edge couplers 1202, 1204. Thedistance, in each loopback alignment feature 1206 or 1208, between itstwo coupling points 1212, 1214 or 1216, 1218 is equal to the distance xbetween the coupling points 1220, 1222 of the pair of waveguide edgecouplers 1202, 1204. The relative positions of the waveguide edgecouplers 1202, 1204 and the loopback alignment features 1206, 1208 areknown.

FIGS. 13A-13C are schematic top views of the photonic chip 1200 of FIG.12 and an optical connector 110 with two communication channels 112, 114successively aligned with the first loopback alignment feature 1206, thesecond loopback alignment feature 1208, and the waveguide edge couplers1202, 1204 of the PIC. Just as with the alignment process for gratingcouplers, the vector {right arrow over (r)}₁ by which the opticalconnector 110 is moved from alignment with the first U-turn waveguideloopback alignment feature 1206 into alignment with the second U-turnwaveguide alignment feature 1208 serves to calibrate the coordinatesystem in which the waveguide edge couplers 1202, 1204 and loopbackalignment features 1206, 1208 are defined. Based on the known vector{right arrow over (r)}₂ between the midpoint between coupling points1216, 1218 of the second loopback alignment feature 1208 and themidpoint between coupling points 1220, 1222 of the waveguide edgecouplers 1202, 1204, the optical connector 110 can then be moved intoits final position, where the optical communication channels 112, 114are connected to the coupling points 1220, 1222 of the waveguide edgecouplers 1202, 1204.

In the embodiments of the above-described figures, the loopbackalignment feature(s) are oriented parallel to a pair (or a row in anarray) of input/output couplers of the respective PICs, such that theoptical connector, following alignment with the loopback alignmentfeature(s), can simply be translated in one or two dimensions into aposition aligned with the PIC input/output couplers. In general,however, the loopback alignment feature(s) may be rotated relative tothe PIC input/output couplers about an axis normal to the plane in whichthe input/output couplers and loopback alignment feature(s) are defined(e.g., the top surface of the photonic chip in surface-couplingembodiments and the side face in edge-coupling embodiments). In otherwords, the line connecting the input/output couplers of the PIC to whichthe optical connector is to be coupled may generally enclose any zero ornon-zero angle with the line connecting the coupling points of aloopback-alignment feature. In accordance herewith, this angle is knownby design. For non-zero angles, movement of the optical connector from aposition aligned with the loopback alignment feature to a positionaligned with the PIC input/output couplers requires a third, rotationaldegree of freedom in addition to the generally two translational degreesof freedom.

FIG. 14 summarizes, in the form of a flow chart, methods 1400 ofaligning an optical connector to input/output couplers of a PIC inaccordance with various embodiments. The methods 1400 begin with theactive alignment of the optical connector with a loopback alignmentfeature formed in the substrate of the photonic chip (act 1402). Thisactive alignment is performed using a light source and detector externalto the photonic chip. Light from the light source is coupled into afirst communication channel of the optical connector, and the detectoris placed to measure light received through a second communicationchannel. The first and second communication channels are initiallyroughly aligned with, and thereby optically coupled to, two couplingpoints of the loopback alignment feature to provide an optical path fromthe light source to the detector. The alignment is then fine-tuned tooptimize the optical coupling and thereby maximize the intensity of thelight measured by the detector. In some embodiments, the opticalconnector is moved to, and the active alignment process is repeated for,a second loopback alignment feature (act 1404). Following the activealignment to the loopback alignment feature(s), the optical connector ismoved (more specifically, translated in one or two dimensions and, ifapplicable, rotated) to a position aligned with the input/outputcouplers of the PIC (act 1406). In that position, the optical connectoris locked in place in any of a number of ways known to those of ordinaryskill in the art, e.g., by gluing with epoxy, soldering, or laserwelding the optical connector the to the input/output couplers (act1408). The methods 1400 may be employed in a manufacturing line toserially align optical connectors to multiple respective PICs. Sinceactive alignment is, in accordance with the methods 1400, limited toalignment features unconnected to the PIC, obviating the need to powerup the PIC, the throughput can be substantially increased, as comparedwith methods for actively aligning optical connecters directly to theinput/output couplers of the PIC.

FIG. 15 illustrates a method 1500 for manufacturing a photonic chip withloopback alignment features as described herein. The method involvescreating a layout for the photonic chip that jointly defines the PIC andloopback alignment feature(s) (act 1502). The photonic chip isphotolithographically patterned in accordance with this layout (act1504), and the patterned chip is processed, e.g., using wet etching ordry etching as known to those of ordinary skill in the art, to createthe photolithographically defined features (act 1506). In someembodiments, patterning and processing steps, and/or the intermittentdeposition of additional layers, are iterated to form more complexlayered structures. The input/output couplers of the PIC and theloopback alignment features are, in various embodiments, patterned onthe chip in a single photolithographic step using a single photomask,which ensures their accurate relative positioning. Even if the PICinput/output couplers and the loopback alignment features are createdwith different photomasks, however, alignment accuracies of these masksrelative to the chip, and the resulting positional accuracies of thepatterned PIC input/output couplers and loopback alignment feature(s),are generally better than 300 nm, enabling the optical-connectoralignment accuracies desired for single-mode coupling.

The input/output couplers of the PIC and the couplers of the loopbackalignment features may all be formed by silicon grating couplers, orthey may all be formed by silicon-nitride-(SiN)-based orindium-phosphide-(InP)-based grating couplers. Alternatively, theinput/output couplers of the PIC may be formed of SiN while the couplersof the loopback alignment features may be formed of silicon or InP, orvice versa. Similarly, edge-coupling PICs may have the input/outputcouplers and the couplers of the loopback alignment features all formedby SiN waveguides and facets, or all formed by silicon waveguides andfacets or InP waveguides and facets. Alternatively, the input/outputcouplers may be formed by SiN waveguides and facets while the couplersof the loopback alignment features may be formed by silicon or InPwaveguides and facets, or vice versa. In any of these examples, the PICand loopback alignment features may be formed on silicon substrates withburied oxide, or on InP substrates, as non-limiting examples.

Having described different aspects and features of loopback alignmentfeatures and associated alignment and manufacturing methods, thefollowing numbered examples are provided as illustrative embodiments.

1. A method comprising: actively aligning an optical connector with afirst loopback alignment feature formed in a substrate of a photonicchip by coupling light from a light source external to the photonic chipvia a first channel of the optical connector into the first loopbackalignment feature and measuring light received from the first loopbackalignment feature via a second channel of the optical connector with adetector external to the photonic chip, the first loopback alignmentfeature being optically unconnected to a photonic integrated circuit(PIC) formed in the substrate of the photonic chip; actively aligningthe optical connector with a second loopback alignment feature formed,unconnected to the PIC, in the substrate of the photonic chip bycoupling light from the light source via the first channel of theoptical connector into the second loopback alignment feature andmeasuring light received from the second loopback alignment feature viathe second channel of the optical connector with the detector; followingactive alignment of the optical connector with the first and secondloopback alignment features, moving the optical connector, based onknown positions of the first and second loopback alignment featuresrelative to input/output couplers of the PIC, to a position aligned withthe input/output couplers of the PIC; and locking the optical connectorin place in the position aligned with the input/output couplers of thePIC.

2. The method of example 1, wherein the first loopback alignment featureand the second loopback alignment feature form separate, opticallyunconnected structures.

3. The method of example 1, wherein the first loopback alignment featureand the second loopback alignment feature form a single structure.

4. The method of example 3, wherein the single structure comprises threegrating couplers optically connected to each other, a distance between afirst one of the three grating couplers and a second one of the threegrating couplers being equal to a distance between the second one of thethree grating couplers and a third one of the three grating couplers.

5. The method of any one of examples 1-4, wherein aligning the firstloopback alignment feature comprises aligning the first and secondchannels of the optical connector with respective first and secondcouplers of the first loopback alignment feature, a distance between thefirst and second couplers being equal to a distance between two of theinput/output couplers of the PIC.

6. The method of any one of examples 1-5, wherein the input/outputcouplers of the PIC comprise at least one of grating couplers or turningmirrors.

7. The method of example 6, wherein the first loopback alignment featurecomprises two couplers optically connected to each other, the twocouplers comprising grating couplers or turning mirrors.

8. The method of example 6, wherein the first loopback alignment featurecomprises two grating couplers optically connected to each other to forma closed loop.

9. The method of any one of examples 1-5, wherein the input/outputcouplers of the PIC comprise waveguide edge couplers.

10. The method of example 9, wherein the first loopback alignmentfeature comprises a waveguide U-turn terminating at a same surface ofthe PIC as the waveguide edge couplers.

11. The method of any one of examples 1-9, wherein the channels of theoptical connector are input/output couplers of a second photonicintegrated circuit (PIC) formed in a second photonic chip.

12. The method of any one of examples 1-11, wherein the active alignmentof the optical connector with the first and second loopback alignmentfeatures is performed without supplying electrical power to the PIC.

13. A photonic chip comprising: a substrate; a photonic integratedcircuit (PIC) comprising a plurality of photonic devices formed in thesubstrate, the plurality of photonic devices comprising a firstplurality of input/output couplers arranged in an array having aconstant pitch; and first and second loopback alignment features formedin the substrate, the first and second loopback alignment features beingoptically unconnected to the PIC and each comprising two couplers, adistance between the two couplers being equal to an integer multiple ofthe constant pitch of the array of input/output couplers of the PIC.

14. The photonic chip of example 13, wherein the first loopbackalignment feature and the second loopback alignment feature formseparate, optically unconnected structures.

15. The photonic chip of example 13, wherein the first loopbackalignment feature and the second loopback alignment feature formportions of a single structure.

16. The photonic chip of any one of examples 13-15, wherein theinput/output couplers of the PIC comprise two grating couplers and thefirst and second loopback alignment features each comprises two gratingcouplers optically connected to each other.

17. The photonic chip of any one of examples 13 and 14, wherein theinput/output couplers of the PIC comprise waveguide edge couplers andthe first and second loopback alignment features each comprise awaveguide U-turn terminating at a same surface of the PIC as thewaveguide edge couplers.

18. A method of manufacturing a photonic chip comprising two loopbackalignment features, the method comprising: photolithographicallypatterning a substrate to simultaneously define, with a singlephotomask, a plurality of input/output couplers of a photonic integratedcircuit (PIC) and the two loopback alignment features, the plurality ofinput/output couplers being arranged in an array having a constantpitch, the two loopback alignment features being optically unconnectedto the PIC and positioned at specified locations relative to a pluralityof input/output couplers, each of the two loopback alignment featurescomprising two couplers, a distance between the two couplers being equalto an integer multiple of the constant pitch of the array ofinput/output couplers; and processing the patterned substrate to createthe photonic integrated circuit and the two loopback alignment featuresin the substrate.

19. The method of claim 18, wherein the two loopback alignment featurescomprise grating couplers.

20. The method of claim 18, wherein the two loopback alignment featuresform a single structure.

Although the inventive subject matter has been described with referenceto specific example embodiments, it will be evident that variousmodifications and changes may be made to these embodiments withoutdeparting from the broader spirit and scope of the inventive subjectmatter. Accordingly, the specification and drawings are to be regardedin an illustrative rather than a restrictive sense.

What is claimed is:
 1. A photonic chip comprising: a substrate; aphotonic integrated circuit (PIC) comprising a plurality of photonicdevices formed in the substrate, the plurality of photonic devicescomprising a first plurality of input/output couplers arranged in anarray having a constant pitch; and at least one loopback alignmentfeature formed in the substrate optically unconnected to the PIC, the atleast one loopback alignment feature comprising a closed loop formed byat least two couplers and at least two waveguides connecting the atleast two couplers, a distance between the at least two couplers beingequal to an integer multiple of the constant pitch of the array ofinput/output couplers of the PIC.
 2. The photonic chip of claim 1,wherein the at least one loopback alignment feature comprises a firstloopback alignment feature comprising the closed loop and a secondloopback alignment feature, optically unconnected to the first loopbackalignment feature, comprising two additional couplers connected to eachother by at least one waveguide, a distance between the two additionalcouplers being equal to an integer multiple of the constant pitch of thearray of input/output couplers of the PIC.
 3. The photonic chip of claim1, wherein the closed loop is formed by at least three couplers and atleast three optical waveguides connecting the at least three couplers,and wherein the at least one loopback alignment feature comprises twoloopback alignment features comprising two respective pairs of couplersamong the at least three couplers.
 4. The photonic chip of claim 1,wherein the at least three couplers are uniformly spaced in a lineararrangement.
 5. The photonic chip of claim 1, wherein the at least twocouplers of the at least one loopback alignment feature comprise gratingcouplers.
 6. The photonic chip of claim 1, wherein the at least twocouplers of the at least one loopback alignment feature comprise turningmirrors.
 7. A method for aligning an optical connector with input/outputcouplers of a photonic integrated circuit (PIC) formed in a photonicchip, the method comprising: actively aligning the optical connectorwith a loopback alignment feature formed in the photonic chip opticallyunconnected to the PIC by coupling light from a light source external tothe photonic chip via a first channel of the optical connector into afirst coupler of the loopback alignment feature and measuring the lightat a second coupler of the loopback alignment feature via a secondchannel of the optical connector with a detector external to thephotonic chip, the light transmitted from the first coupler to thesecond coupler via two separate optical connections each comprising awaveguide; after active alignment of the optical connector with theloopback alignment feature, moving the optical connector, based at leastin part on a known positions of the loopback alignment feature relativeto the input/output couplers of the PIC, to a position aligned with theinput/output couplers of the PIC; and locking the optical connector inplace in the position aligned with the input/output couplers of the PIC.8. The method of claim 7, further comprising actively aligning theoptical connector with an additional loopback alignment feature formedin the photonic chip optically unconnected to the PIC, and calibrating acoordinate system in which the PIC and the loopback alignment featuresare defined relative to a movement of the optical connector based on aknown relative position between the loopback alignment features.
 9. Themethod of claim 8, wherein the loopback alignment feature and theadditional loopback alignment feature form a single closed loopcomprising the first coupler, the second coupler, and a third coupler ina linear arrangement.
 10. The method of claim 8, wherein the loopbackalignment feature and the additional loopback alignment feature formseparate, optically unconnected structures.
 11. The method of claim 7,wherein the optical connector is aligned with the input/output couplersof the PIC without supplying electrical power to the PIC.
 12. The methodof claim 7, wherein the optical connector is aligned with theinput/output couplers of the PIC without establishing electricalconnections between an alignment system and the PIC.
 13. The method ofclaim 7, wherein the optical connector is moved into the positionaligned with the input/output couplers of the PIC using a robot withhigh-accuracy translational and rotational stages.
 14. The method ofclaim 7, wherein the optical connector is aligned to the input/outputcouplers of the PIC at a temperature selected based on an intended rangeof operating temperatures of the PIC.
 15. A method of manufacturing aphotonic chip comprising at least one loopback alignment feature, themethod comprising: photolithographically patterning a substrate tosimultaneously define, with a single photomask, a plurality ofinput/output couplers of a photonic integrated circuit (PIC) and the atleast one loopback alignment feature, the plurality of input/outputcouplers being arranged in an array having a constant pitch, and the atleast one loopback alignment feature comprising a closed loop formed byat least two couplers and at least two waveguides connecting the atleast two couplers, a distance between the at least two couplers beingequal to an integer multiple of the constant pitch of the array ofinput/output couplers of the PIC; and processing the patterned substrateto create the photonic integrated circuit and the at least one loopbackalignment feature in the substrate.
 16. The method of claim 15, whereinthe at least two couplers comprise grating couplers.
 17. The method ofclaim 16, wherein the grating couplers comprise at least one ofsilicon-nitride-based or indium-phosphide-based grating couplers. 18.The method of claim 15, wherein the at least one loopback alignmentfeature comprises two loopback alignment features forming a singlestructure.
 19. The method of claim 18, wherein the two loopbackalignment features comprise at least three couplers uniformly spaced ina linear arrangement.
 20. The method of claim 15, wherein the at leastone loopback alignment feature comprises two loopback alignment featuresforming separate, optically unconnected structures.